NEW YORK (Reuters Health) - The aging process can take a toll on the body's internal rhythms, and now new research suggests that some body parts age faster than others--at least when it comes to telling time.

In a study of old and young rats, internal "clocks" in certain tissues became less reliable with age, but others "kept time" as well as they did in younger animals.

For example, the lungs appear to lose their 24-hour rhythm, and half of the older rats had a random pattern of gene activity compared with their younger counterparts. In contrast, the liver stayed on target, churning out proteins in a cycle that matched the rest of the body.

And in certain tissues--such as the kidney--a 24-hour cycle was maintained but pushed slightly forward. For example, the peaks and valleys of gene activity in the elderly rats were 4.5 hours ahead of the younger animals.

The findings are published online in the Early Edition of the Proceedings of the National Academy of Sciences.

The body's internal clock, or circadian rhythm, controls when we sleep and wake and plays a role in other biological processes as well, such as temperature, hormone production and blood pressure. The body's main clock is located in a part of the brain called the hypothalamus, but other "peripheral" clocks are located in other tissues throughout the body.

That circadian rhythms change with age is not a surprise, but scientists have not known whether the alterations are caused by changes in the central clock in the brain, the peripheral clocks in tissues or in the connections between the two types of internal clocks.

In the new study, the researchers used rats genetically engineered to contain a firefly gene. The researchers then measured circadian rhythms by looking at how the tissue glowed when the rats' cells were exposed to luciferin, a bioluminescent compound found in insects.

Overall, the study "suggests that some parts of the molecular clock within the hypothalamus are not affected much by aging," lead investigator Dr. Gene D. Block, of the University of Virginia in Charlottesville, told Reuters Health.

But Block pointed out that the study also suggests that peripheral clocks in some tissues, such as the lungs, are affected by aging.

"This was unknown until this study," Block said.

Finally, Block said that the study shows that some peripheral clocks are not being stimulated properly in order to keep running as they should. He noted that some peripheral clocks must be stimulated at least every few days to stay on schedule.

For example, the researchers could stimulate lung tissue to get back on its "regular schedule" by exposing the tissue to certain triggers.

"It appears that aging may cause a breakdown in the coordination between the central clock in the hypothalamus and some body rhythms," the Virginia researcher said. "In this sense aging is causing effects on the clock system rather than on the basic molecular machinery of the central clock in the hypothalamus."

SOURCE: Proceedings of the National Academy of Sciences Early Edition 2002;10.1073/pnas.152318499.

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